1. Field of the Invention
The present invention relates to electrooptic display devices which modulate the light passing through them and more precisely to liquid-crystal panels.
These panels have useful characteristics for producing display screens in avionics: they are less bulky than conventional cathode-ray tube screens and they consume less power.
2. Discussion of the Background
On a liquid-crystal panel, an image is displayed by means of juxtaposed colored, or black, elementary dots. An elementary dot corresponds to light transmitted to its front face via a liquid-crystal cell illuminated on its rear face. A liquid-crystal cell generally comprises, from the rear to the front, a stack consisting of a polarizer, a first transparent substrate, a thin liquid-crystal layer, a second transparent substrate and an analyzer.
These transparent substrates include electrodes which are also transparent and when a voltage is applied to them this allows the liquid-crystal molecules to be subjected to an electric field perpendicular to the plane of the cell.
A liquid-crystal molecule has two remarkable characteristics; first, depending on its spatial orientation, it can modify the polarization of light passing through it and, secondly, an electric field can change its orientation.
Thus, the liquid-crystal layer at rest (with no voltage applied) and said layer with voltage applied lead to two distinct arrangements of the liquid-crystal molecules in the layer, defining two states of the cellxe2x80x94the activated state and the unactivated statexe2x80x94such that, for example, in one state the cell lets light through and in the other state it absorbs it. Depending on its state, the cell allows a white dot or a black dot to be displayed. Grays can be produced with intermediate voltages which impose other orientations on the liquid-crystal molecules. In addition, a color filter inserted into the stack of a cell allows a colored dot to be presented. This technology is used for displaying black-and-white or color images.
However, with a liquid-crystal panel the image perceived depends on the viewing angle at which the panel is observed. A high-quality image perceived by the observer when he looks along the direction normal to the plane of the panel is distorted when the direction at which he looks is inclined with respect to this normal direction. This in general reduces the use of a liquid-crystal panel to directions of observation departing little from the normal to the panel: the panel has a restricted viewing angle.
For a display screen which has to be able to be read by an observer whose position is not fixed and/or by several observers placed around the screen, such as, for example, a screen in the flight deck of an aircraft, the restriction in viewing angle of the liquid-crystal panel is a serious drawback.
For the user of the screen, the liquid-crystal layer has drawbacks in optical behavior. The drawbacks are especially due to the light contrast between the states of a cell which has the annoying feature of varying with the angle of observation and therefore of disrupting the observation of the panel.
This effect is explained by the natural birefringence of a liquid-crystal molecule whereby the modification by the molecule of the polarization of the light passing through it depends on the relative orientation between this light and the molecule and the change in angle of observation modifies the polarization of the light received by the analyzer and therefore modifies the transmission of the cell.
The prior art proposes partial corrections of this birefringence in certain situations in which it is a problem for various types of liquid-crystal cell.
We are interested more particularly in cells comprising a twisted nematic liquid crystal and mutually crossed polarizers (a polarizer and an analyzer) lying on either side of the liquid-crystal layer. In their unactivated state, these cells ensure that there is a high transmission of the light received. Their activated state corresponds to strong absorption of the light; an activated cell observed along its normal ensures that the light transmission is very low. The main drawback with these cells is, in the activated state, a marked increase in the light transmission when observed at an angle inclined with respect to the normal to the cell: a black dot observed normal to the cell becomes lighter when the observer moves away from the normal to the cell and the contrast between white and black decreases with this movement away from the normal. The contrast is the ratio of the transmissions in each state, i.e. the ratio of the transmission of the cell in the activated state to that in the unactivated state.
In the activated state, the birefringence of the liquid-crystal molecules is penalizing.
The prior art proposes to correct, always incompletely, this birefringence by adding a birefringent compensating film to the stack of, the cell.
A first known correction consists in using a uniaxial birefringent film having a negative refractive index anisotropy in the direction perpendicular to the plane of the cell. The film is a negative uniaxial film with an extraordinary optical axis normal to the cell.
Thereafter, corrections giving greater satisfaction have been developed. Thus, for example, patent EP 0 646 829 proposes a birefringent correcting film comprising a support having the characteristics of the first known correction and on which a discotic liquid crystal oriented by rubbing is polymerized. It describes a T type of birefringent film comprising a stack of two negative uniaxial media each having an extraordinary optical axis, one being parallel to the normal to the cell and the other being inclined with respect to this normal on the one hand and with respect to the plane of the cell on the other.
The problem consists in how to further widen the field of observation of a liquid-crystal panel comprising, in the stack of the cell and on each side of the crystal layer of the cell, at least two negative uniaxial media such that the extraordinary axis of one of these media is parallel to the normal to the plane of the cell and the extraordinary axis of another of these media is inclined both with respect to this normal and with respect to the plane of the cell.
Furthermore, especially in the case of a panel, such as those used for example in civil avionics, placed vertically, it is sought to further improve the viewing angle, particularly along the horizontal.
The invention provides a novel solution which consists in adding, to the stack of the liquid-crystal cell, a positive uniaxial birefringent compensating film to the birefringent films of the T type above.
More specifically, the invention provides a display device with a liquid-crystal cell comprising a twisted nematic liquid-crystal layer placed between two crossed polarizers and including, between the liquid-crystal layer and at least one of the polarizers, a structure for compensating for the variations in contrast of the cell as a function of the angle of observation, which structure comprises at least two superposed negative uniaxial media such that the extraordinary optical axis of one medium is parallel to the normal to the cell and that of the other medium is inclined with respect to this normal on the one hand and with respect to the plane of the cell on the other, characterized in that the compensating structure furthermore includes a layer of a positive uniaxial birefringent material.
In a known manner, a positive uniaxial material may be compensated for by a negative uniaxial material. Furthermore, a liquid-crystal molecule may be considered as behaving approximately like a stack of positive uniaxial materials. The liquid-crystal layer can therefore reasonably be compensated for by a stack of uniaxial materials having a negative birefringence, i.e. having a birefringence of opposite sign to the intrinsic birefringence of the liquid crystal.
The prior art teaches how to compensate for the positive birefringence of the molecules of the crystal layer by a particular stack of two negative uniaxial materials in a T-type film.
The novel solution of the invention consists in improving the compensation for the positive birefringence of the molecules of the liquid-crystal layer by supplementing the particular stack of the prior art with an additional layer of material whose birefringence is not of the opposite sign to that of the liquid crystal but, like it, has a positive birefringence.
The material of the additional layer is a positive uniaxial birefringent material. The extraordinary axis of this material is approximately parallel to the plane of the liquid-crystal layer. The axis is directed so as to be approximately perpendicular to the direction of alignment of the molecules on the face closest to the liquid-crystal layer.
In the thickness of the cell, the relative orientations of she twisted crystal layer, of the pair of polarizers and of the extraordinary optical axes of the second medium of the compensating structure and of the material of the additional layer maximize the viewing angle of the device in a plane perpendicular to the plane of the cell.
The additional layer preferably lies between the liquid-crystal layer and the T-type compensating film; the additional layer lies in the immediate vicinity of the crystal layer.
In the liquid-crystal layer, the directions of alignment of the molecules on the faces of the layer are preferably perpendicular.
Preferably, each polarizer is crossed with the alignment direction of the face closest to the liquid-crystal layer. In addition, the extraordinary axis of the material of the additional layer is crossed with the closest polarizer.
The invention makes it possible to appreciably widen the field of observation of a liquid-crystal panel by improving the level of the contrast, without introducing contrast inversion. In particular, it makes it possible to obtain a vertical screen that can be used with a very high viewing angle along the horizontal, typically plus or minus 60 degrees and quite a large viewing angle in the vertical direction, typically up to 40 degrees for an observer reading a panel placed lower than his face.
The vertical screen has both an absence of contrast inversion and a contrast greater than 40 for an angle of observation lying within an angular range, defined with respect to the straight line normal to the screen, of between +60 and xe2x88x9260 degrees along the horizontal and of between 0 and 40 degrees along the vertical above the normal line.